The efficient catalytic trimerization or tetramerization of olefinic monomers, such as the trimerization and tetramerization of ethylene to 1-hexene and 1-octene, is an area of great interest for the production of olefinic trimers and tetramers of varying degrees of commercial value. In particular, 1-hexene is a valuable comonomer for linear low-density polyethylene (LLDPE) and 1-octene is valuable as a chemical intermediate in the production of plasticizer alcohols, fatty acids, detergent alcohol and lubrication oil additives as well as a valuable comonomer in the production of polymers such as polyethylene. 1-Hexene and 1-octene can be produced by a conventional transition metal oligomerization process, although the trimerization and tetramerization routes are preferred.
Several different catalytic systems have been disclosed in the art for the trimerization of ethylene to 1-hexene. A number of these catalysts are based on chromium.
U.S. Pat. No. 5,198,563 (Phillips) discloses chromium-based catalysts containing monodentate amine ligands useful for trimerizing olefins.
U.S. Pat. No. 5,968,866 (Phillips) discloses an ethylene oligomerization/trimerization process which uses a catalyst comprising a chromium complex which contains a coordinating asymmetric tridentate phosphane, arsane or stibane ligand and an aluminoxane to produce alpha-olefins which are enriched in 1-hexene.
U.S. Pat. No. 5,523,507 (Phillips) discloses a catalyst based on a chromium source, a 2,5-dimethylpyrrole ligand and an alkyl aluminium activator for use in the trimerization of ethylene to 1-hexene.
Chem. Commun., 2002, 8, 858-859 (BP), discloses chromium complexes of ligands of the type Ar2PN(Me)PAr2 (Ar=ortho-methoxy-substituted aryl group) as catalysts for the trimerization of ethylene.
U.S. Pat. No. 7,141,633 (BP) discloses a catalyst for the trimerization of olefins comprising a source of chromium, molybdenum or tungsten, a ligand containing at least one phosphorus, arsenic or antimony atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, and optionally an activator. The ligand used in most of the examples is (2-methoxyphenyl)2PN(Me)P(2-methoxyphenyl)2.
Although the catalysts disclosed in the BP documents mentioned above have good selectivity for 1-hexene within the C6 fraction, a relatively high level of products other than the commercially desirable 1-hexene and 1-octene is typically observed.
U.S. Pat. No. 7,273,959 (Shell) discloses a trimerization catalyst composition and a process for the trimerization of olefinic monomers using said catalyst composition.
Catalytic systems for the tetramerization of ethylene to 1-octene have recently been described. A number of these catalysts are based on chromium.
U.S. Published Patent Applications Nos. 2006/0128910, 2006/0173226, 2006/0211903, and 2006/0229480 (Sasol) disclose catalyst compositions and processes for the tetramerization of olefins. The catalyst compositions disclosed comprise a transition metal and a heteroatomic ligand having the general formula (R)nA-B-C(R)m where A and C are independently selected from a group which comprises phosphorus, arsenic, antimony, oxygen, bismuth, sulphur, selenium, and nitrogen, and B is a linking group between A and C, and R is independently selected from any homo or heterohydrocarbyl group of which at least one R group is substituted with a polar substituent and n and m are determined by the respective valence and oxidation state of A and/or C. The other catalyst compositions disclosed comprise a transition metal and a heteroatomic ligand having the general formula (R′)nA-B-C(R′)m where A, B, C, n and m are as defined above, and R′ is independently selected from any homo or heterohydrocarbyl group.
U.S. Published Patent Application No. 2006/0128910 (Sasol) discloses the tandem tetramerization and polymerisation of ethylene. Specifically, it discloses a process for polymerising olefins to produce branched polyolefins in the presence of a distinct polymerization catalyst and a distinct tetramerization catalyst, wherein the tetramerization catalyst produces 1-octene in a selectivity greater than 30% and the 1-octene produced is at least partially incorporated into the polyolefin chain.
Although the tetramerization catalysts disclosed in the Sasol documents mentioned above have good selectivity for 1-octene within the C8 fraction, however, only about 70 to 80% wt. of the C6 composition is 1-hexene, with the remaining C6 by-product comprising compounds such as methylcyclopentane and methylenecyclopentane. The presence of these other C6 compositions, which have very little commercial use or value, is highly undesirable from both an economic point of view as well as from a product separation point of view.
Heteroatom Chemistry, volume 2, page 477 discloses the preparation of (phenyl)2P—N(isopropyl)-P=catechol and catechol=P—N(isopropyl)-P=catechol. However there is no disclosure in this document of the use of these compounds in catalyst systems for the trimerization and tetramerization of olefins.
It has now surprisingly been found that the catalyst systems derived from the ligands of the present invention are valuable in providing high levels of both hexene and octene in a process for the simultaneous trimerization and tetramerization of ethylene, with a high selectivity for both 1-hexene and 1-octene within the C6 and C8 fractions, respectively. In addition, the catalyst systems of the present invention have improved activity and allow the trimerization/tetramerization reaction to proceed at industrially attractive process conditions (e.g. elevated temperature and pressure) without fast decay of the catalyst.